The present invention is related to carbon fibers, and in particular, fine denier carbon fibers and to processes for making the same.
Carbon fibers consist of strongly bonded basal planes of aromatic rings stacked tightly on top of each other. Carbon fibers may be produced from a variety of precursors. Two of the most important precursors commercially are polyacrylonitrile (PAN) and natural pitch. In general, a series of heat treatments are used to remove the various non-carbon elements contained in the precursors.
Because of their structure, carbon fibers exhibit properties such as high tensile strength or modulus, chemical resistance, flame resistance, and/or low resistivity. As such carbon fibers are widely used in a variety of applications.
For example, carbon fibers are used extensively in composites. Composites are generally defined as synergistic material systems comprised of a combination of two or more constituents differing in form and/or composition which are insoluble in each other. One of the constituents acts as a reinforcement, bearing the loads to which the composite is subjected. A second component is referred to as the matrix, typically a resin, whose function is to transfer the load between the reinforcing elements. An interface is formed between the reinforcement and the matrix constituents, and the adhesion arising at this interface determines the mechanical properties of the composite article as a whole.
Of particular interest are xe2x80x9cadvanced composites,xe2x80x9d which are generally defined as products which are reinforced with materials possessing higher specific tensile strength and/or modulus than the materials they replace, or which contain a matrix exhibiting outstanding temperature or chemical resistance. Advanced composites typically consist of continuous filament reinforcement embedded in a high performance matrix material. Carbon fibers are widely used as the reinforcement constituent in advanced composites.
Although having better modulus, pitch based carbon fibers have lower tensile strength than PAN based carbon fibers, and are generally difficult to melt spin and otherwise process. One technique proposed to address this issue involves the use of bicomponent fibers. For example, U.S. Pat. No. 3,639,953, is directed to bicomponent fibers having a pitch component and a protective synthetic organic component. The synthetic organic polymer component is used to impart strength to the pitch during spinning and to protect the pitch component during later processing. The protective component remains adhered to the pitch component during both melt spinning and subsequent heat treatments, thus becoming an integral part of the final fiber. As stated in the ""953 patent, the synthetic organic component and pitch component become infusible and insoluble to heat and solvent. Because the synthetic organic component remains fused to the pitch component, the resultant fibers would be expected to have limited use in many applications, including composites, because the synthetic organic component would interfere with the properties of the composite.
Although possessing better melt spinning properties, PAN based carbon fibers, in particular high modulus PAN carbon fibers, lack flexibility. Several methods have been proposed to address this deficiency. For example, U.S. Pat. No. 5,518,836, is directed to a process for reducing the formation of a high degree of order or crystallinity within the precursor fiber using sub-micron particles. Such fibers, however, would exhibit inferior tensile properties in comparison to traditional PAN based carbon fibers. U.S. Pat. No. 5,858,530 is directed to the use of biregional PAN carbon fibers, in which only the outer portion of the fiber is carbonized. These fibers also would be expected to suffer inferior tensile properties.
The present invention provides dissociable multicomponent fibers having a melt processable polyacrylonitrile polymer residual component and a fugitive polymer component. Advantageously, the multicomponent fiber is produced by melt spinning a melt processable polyacrylonitrile and fugitive polymer. Following melt extrusion, the melt processable polyacrylonitrile may be subjected to various heat treatments, thus converting the polyacrylonitrile into carbon fibers.
The inventors have found that the multicomponent fibers of the invention can be readily dissociable, yet able to survive conventional textile processing intact. Thus the multicomponent fibers can be used to produce carbon microfilaments having desirable physical properties. In this regard, the fugitive polymer component can be extracted out of the multicomponent fiber at any one of several points of the fiber making process to provide fine denier carbon fibers. Advantageously the resultant carbon microfibers have a denier of less than 0.2 and preferably from about 0.005 to about 0.16 denier.
For example, the fugitive component can include water soluble polymers, such as but not limited to, polyvinyl alcohol, polyethylene oxide, polyacrylamide, polylactic acid, water soluble copolyester resins, and copolymers, terpolymers, and mixtures thereof. Other additives, such as basic or ionic compounds, may be added to an aqueous bath to aid in dissolution, as is known in the art. For example, polylactic acid can be soluble in caustic aqueous solutions. Alternatively the fugitive polymer can include a solvent extractable polymer, such as polystyrene. In this embodiment of the invention, the fugitive polymer is typically extracted from the multicomponent fiber using a suitable solvent after melt spinning but before heat treatment to carbonize the PAN component.
Alternatively, the fugitive component can include a polymer which forms a char upon heat treatment such as that generally associated with the carbonization of PAN polymers. The charred residual component can then be extracted or removed from the multicomponent fibers generally using mechanical means, such as impingement by high pressure air or water jets.
The multicomponent fibers can have a variety of configurations, including pie/wedge fibers, segmented round fibers, segmented oval fibers, segmented rectangular fibers, segmented ribbon fibers, segmented multilobal fibers, and islands-in-the-sea fibers. Thus the resultant carbon microfibers (or carbon microfiber precursors) can in turn have a variety of shapes. Further, because the multicomponent fibers can be meltspun, the resultant fine denier carbon fibers can have desirable strength properties. In addition, the fine denier carbon fibers can exhibit improved flexibility, as compared to conventional carbon fibers. Still further, the fine denier carbon fibers of the invention can possess greater surface area than conventional carbon fibers, and thus can provide improved adhesion in composites and other applications. The fine denier carbon fibers of the invention can also exhibit improved tensile strength as compared to conventional carbon fibers with minimal or no sacrifice of other properties. Still further, the fine denier carbon fibers of the invention can exhibit improved heat insulating properties, and thus can be suitable for use at high temperatures.
The multicomponent fibers of the present invention can be formed into a variety of textile structures, such as fabrics. Fabrics of the present invention may generally be formed by weaving, knitting, or nonwoven processes. In this aspect of the invention, the multicomponent fibers can be dissociated to form microfilaments prior to, during or following fabric formation. The resultant fabrics which include fine denier carbon fibers can be economical to produce and further can have superior characteristics, particularly when used in composites or filtration applications.
The carbon microfibers of the present invention may also be formed into other useful articles, such as yarns, prepreg tape, filtration media, and composites. The yams can include the carbon microfibers of the invention solely, or the carbon microfibers may be commingled with other fibers. The present invention also provides processes for making the multicomponent fibers, carbon microfibers, fabrics and other articles.